I do almost all of my tree clearing and treatments in the fall after the first hard frost - when the sap is flowing back downwards into the roots.
This is good advice for killing any sort of woody brush, whether you're using Tordon on stumps, or a glyphosate/2-4-D/diesel cocktail on foliage.
You don't necessarily need to wait on a hard freeze. Once leaf drop starts you know the sap is headed down.
For the folks who are dead-set against chemical treatments, springtime is the right time to do mechanical removal.
Wait for things to get fully leafed out, then cut it off. Wait for it to sprout back, then cut it off again. Do that enough times and the root will starve.
